One known planar carbon segment commutator, disclosed in the specification of German Utility Model G 89 07 045.3, in the joint names of Deutsche Carbone AG and Robert Bosch GmbH, comprises a base member of insulating material having a rotational axis, a front surface extending, at least in part, transversely to the rotational axis, and a plurality of apertures extending rearwardly from the front surface; a plurality of circumferentially spaced contact members mounted on the front surface; and a plurality of circumferentially spaced overmoulded carbon segments respectively formed on the contact members and each having integral anchor means which extend rearwardly into said apertures.
Although the anchor means provide some radial support for the carbon segments mounted on the contact member, against centrifugal force, and some support against axial withdrawal from the contact member, this support is dependent upon the shear strength of the carbon in the anchor means and on the frictional engagement between the anchor means and the apertures in which they extend. Therefore, to ensure adequate radial and axial support for the contact members and the carbon segments, the contact members have to be provided with rearward extensions insert moulded into the front surface of the base member and undercut recesses for receiving integral rear portions of the overmoulded carbon segments. This involves relatively complex and time-consuming forming and pre-assembly operations. Moreover, the resultant products are not particularly robust.